Invasive Species: Invited But Unwanted

A new decision-making process could help curb the devastating effects of invasive plants on natural environments in the U.S.

This article was published 20 years ago — in the July 1, 1996 issue, to be exact — and upon learning of the passing of author Sarah Reichard, we thought it was appropriate to open the dialogue again. We’ve come a long way; we’ve a long way to go.

Several years ago a popular bumper sticker proclaimed, “Free Gondwanaland!” referring to the now-separated supercontinent that once united Africa, South America. Australia and India. While the bumper sticker was obviously facetious, we have already biologically united not only Gondwanaland, but also Laurasia – the North American, European and Asian supercontinent.

Over the last few centuries, we have intentionally and unintentionally moved thousands of species into areas of the world where they, or even species like them, have never before occurred. One of the most common reasons for introducing plant species into new areas is for ornamental or landscape purposes. With these introductions, we have often set into motion interactions between species that we could not have predicted and cannot control.

Probably more than 5 percent (105 million acres) of our nation’s lands and waters are now overrun by invasive, non-native plants. This includes 475,000 acres of wetlands taken over by Lythrum salicaria (purple loosestrife) across the northern third of the country, 1.5 million acres of rare wetlands and riparian areas in the West being literally dried out by Tamarix (tamarisk or salt cedar), and many others. Not included in that figure are the many acres of forests that are significantly altered by non-native invaders. Within nearly 200 of the approximately 250 National Parks protecting significant natural resources, non-native plants have been identified as serious threats to those resources. Stewards of 60 percent of The Nature Conservancy’s preserves report plant invasions to be significant threats to native organisms and ecosystems.

Biologists are only just beginning to understand the dynamic interactions between native and non-native species that have been set in motion by biologically reuniting Gondwanaland and Laurasia. And, it is unlikely that this “biological pollution” has reached its most damaging level. It is clear, however, that if we do not move from our current reactive approach to invasions of natural areas to a proactive one of preventing and controlling new introductions, we will continue to lose our natural areas and endanger native species.

Nursery professionals are quite familiar with the impact of weeds in managed landscapes. It is, therefore, easy to recognize the impact that weeds can have on natural landscapes, and empathize with the frustration of trying to deal with the management of weeds on the scale of millions of acres.

It is better to try to prevent a weed problem than to control it. One possible solution is to evaluate the invasive potential of woody plants before they are introduced. A decision chart, which is presented on page 22, provides a method for nursery professionals to determine the invasive potential of exotic plants before they become aggravating, and expensive, pests.

Entering at Our Own Risk

Many of the current plant invaders in North America entered accidentally – often as crop-seed contaminants, in soil used on ships for ballast and on (or in) livestock. Improved screening procedures, the Federal Seed Act, better crop-seed collection methods and ceasing the use of dry ballast have greatly reduced the number of species that enter accidentally. Inadvertent entry, though, is still common.

Intentional introductions are also an important source of invasive species. For instance, research shows that of the 235 woody plant known to invade natural areas in the U.S., 85 percent were introduced primarily for ornamental and landscape purposes, while another 14 percent were introduced for agricultural uses. There are few regulations governing international introductions and the flow of new species into the country, although less than in previous decades, is steady. Transportation of current and potentially invasive plant species also actively continues within the U.S.

Some long-standing plant-distribution practices may make the spread of new invasive species ultimately uncontrollable, but ending those practices could have an impact on lessening the extent of invasion. For instance, some importers exchange seed or plantlets of newly introduced species in return for financial assistance for collecting expeditions. The plants are usually distributed to supporters soon after importation, when the importer knows little about the species’ invasive potential.

Studies have shown that when an invasive species is broadly distributed and dispersing its offspring from scattered populations, control is nearly impossible. Single, large populations however, are often controllable. The figures on this page illustrate how this can work. The large population has less exterior surface area and disperses seed back into the existing population. The several smaller ones, on the other hand, have a greater total surface area and disperse mostly outward into new territory because of the small-size population.

One large, invasive plant population has less exterior surface area and disperses seed back into the existing population, making it easier to control than several smaller populations that have a greater exterior surface area and disperse seed outward into new territory.

Currently, U.S. efforts to counter the effects of non-native invasive species are a “largely uncoordinated patchwork of laws, regulations, policies and programs,” according to a 1993 congressional report. The Federal Noxious Weed Act is the main legislation restricting the entry and spread of plant species into the U.S. If someone tries to enter the country with one of the listed species, the plant will be confiscated and the importer risks legal penalties.

The most fundamental problem with the act is that, historically, a species is listed only after it has already become widespread, done substantial damage and become essentially impossible to control. Other problems with the act include a lack of application to pests of natural areas and a lack of procedures for emergency listing of species while they are in an early phase of establishment.

The act also controls only a small percentage of invasive plant species. It is estimated that there are 750 species that meet the definition of a noxious weed, but only 94 species are listed. Other laws dealing with non-native species include the Federal Seed Act and various state noxious-weed acts. A number of agencies also have policies and regulations regarding these pests.

Strong recommendations have been made to shift the responsibility of costs for harmful introductions from taxpayers to those who benefit from them, such as nursery professionals who import and sell them. Given that the removal cost of only one species, Melaleuca quinquenervia (paperbark tree), was $1.3 million in 1991, sharing that responsibility could be crippling. Voluntary efforts to reduce the introduction of invasive plants will likely make such restrictions less of a probability in the future.

A comprehensive and proactive policy would attempt to establish a species’ invasive potential before it is too widespread to be effectively controlled, and would consider pests of natural areas equal to those of commodity-managed lands, including agricultural or rangelands. Perhaps the most difficult aspect of changing from a reactive approach, such as the Federal Noxious Weed Act, to a proactive one is determining a plants’ invasive potential either before its introduction or before the species becomes widespread. Recent research has made some progress, however, on how such evaluations can be made.

The Decision Chart

Nursery professionals can use a decision chart for evaluating the invasive potential for woody plants. The chart was developed after extensive comparisons of 235 currently invasive woody-plant species and 114 species that, in spite of long cultivation in the U.S., do not demonstrate invasive behavior. It is based on the findings that high proportions of the species that are invasive in the U.S. share certain traits that are not shared with the noninvasive species.

These findings reveal several important facts. Fifty-four percent of the woody species that invade the U.S. also invade other parts of the world. In terms of growth and expansion, 44 percent spread by vegetative means, including rhizomes, root suckers and soil layering. These species have shorter juvenile periods – from the time of germination to flowering and fruiting. And 51 percent of the invasive woody species do not require any pretreatment of the seed to induce germination. Very few invaders are introduced from other parts of North America (3 percent) or are interspecific hybrids (1 percent). These traits may be directly affecting invasive ability, or they may be linked to another trait and serve as indicators, rather than causes.

The noninvasive plants, on the other hand, rarely invade elsewhere (15 percent) or spread by vegetative means (23 percent). The juvenile period is, on average, three years longer, and only 30 percent do not require pretreatment to induce germination. Twenty-five percent of the noninvaders are from other parts of North America, and 11 percent are interspecific hybrids.

The information needed to complete the chart can generally be found in a well-equipped library, especially one at a university. Information about a species’ invasive potential can be found in plant manuals and floras from areas with similar climates, including Australia, New Zealand, South Africa and Europe. You can also find information by searching abstract services on CD-ROM, such as AGRICOLA and the Life Sciences Collections. Efforts are being made to get lists of invaders linked on the World Wide Web, which will make discovering where species invade even easier.

Information about a species’ origin, hybridization, vegetative reproduction and seed requirements may be found in horticultural texts and publications. Even seemingly obscure species have often been studied by government agencies for some economic use, and the resultant publications may be found through computerized abstract searches. In some cases, observing plants of the species may be necessary.

The chart has three outcomes: accept (there is a low probability of invasiveness), reject (there is a high probability of invasiveness) and evaluate and monitor further. The latter category means that there are indications that the species has invasive potential and that holding it for further observations would be prudent.

In a test of the chart using the 349 species studied, no invaders would be accepted, 89 percent would be rejected and 11 percent would need further monitoring. Of the noninvaders, 38 percent would be accepted, 24 percent would be rejected (mostly because they invade elsewhere), and 39 percent would need further monitoring. Twenty-four percent is a high rejection level for noninvaders and one that can likely be reduced in the future with more information. In the meantime, it is a trade-off – allowing the protection of natural areas for the cautious importation of species.

The chart is thus most effective in restricting the entry of invasive species, which is the key objective. It is less effective in allowing free entry to noninvaders. It essentially encourages the use of superior cultivars of North American natives and nonvegetatively reproducing hybrids. It is cautious about other introductions, suggesting monitoring and further evaluations to all other species. Increasing information about what makes a species noninvasive should increase free entry to more species. The chart is also limited in that it applies to woody species only; similar charts must be developed for herbaceous perennials and annuals.

It is unfortunate that many of the characteristics that are associated with invasiveness are ones that could be desirable to nursery and landscape applications. For instance, quick vegetative reproduction could be desirable for stabilizing soil, and rapid attainment of reproductive status would mean the ability to sell blooming plants more quickly. In the former case, planting more plants of noninvasive species or planting vegetative reproducers only in areas where they will be actively managed and contained could alleviate the problem. In the latter case, using cuttings rather than seeds when possible, would shorten the period to flowering in the nursery (there is often a substantial difference in length of time to blooming for cuttings and seeds). A few blooming plants could also be displayed with younger, nonblooming plants in a nursery so that the customer would know what to expect for flowers. With planning and care, it is entirely possible not to affect operations and still protect natural areas from aggressive and harmful species.

The land-managing agencies will never have adequate funds to fully protect lands when surrounding areas continue to be infested. To protect parks, wildlife refuges and public lands, resource managers will need the active cooperation of those responsible for introducing new species, those managing neighboring lands and water, as well as sensible national policies that effectively exclude as many invasive alien species as possible.

Knowing that the majority of environmental weeds are escaped ornamental plants should encourage nursery professionals to use available information and share their practical knowledge about weeds so that future introductions will not negatively impact native species. We must create and coordinate new barriers, including restrictive policies, to replace the geographic barrier that human technology has overcome. And that’s something you can’t put on a bumper sticker.

Facts on Alien Attacks

In the U.S. there are at least 2,000 non-native plants invading managed and natural systems.

Of those, at least 235 woody plants and at least 600 herbaceous plants, including grasses and aquatic species, are known to invade natural areas.

To date, only about 15 percent of potential weeds in the world have had an opportunity to invade the U.S.

Vines such as Lonicera japonica (Japanese honeysuckle) and Hedera helix (English ivy) have been shown to inhibit regeneration of trees and native wildflowers by blanketing the ground and denying light to young plants.

Peuraria lobata (kudzu) and Euonymus fortunei (winter creeper) also outcompete native species for light in the Southeast.

Tamarix (tamarisk) not only can change the course of rivers by trapping sediment, but it also removes large volumes of water from the soil and denies water to native species by lowering the water table below their reach.

Eucalyptus (eucalyptus), which is adapted to survive and even facilitate fires, has been implicated as an aggravating factor in the fires in the Berkeley Hills of California in 1991 that killed 25 people and resulted in a loss of more than $1.5 billion.

Lythrum salicaria (purple loosestrife) threatens to infest 90 percent of the freshwater wetlands that produce waterfowl found in the Atlantic and Mississippi waterways.

About the AuthorDr. Sarah Reichard and Dr. Faith Campbell

Dr. Sarah Reichard received her Ph.D. from the University of Washington’s Center for Urban Horticulture in 1994. At the time of her passing in August, she was Director of the University of Washington Botanic Gardens. Dr. Faith Campbell received her Ph.D. in political science from Princeton University. Among many other positions she has held since publication of this article, she currently is Vice President of the Center for Invasive Species Prevention, which can be accessed at www.cisp.us. Dr. Campbell can be reached at phytodoer@aol.com.